In many applications, including communications systems, satellite-based navigation systems, and other applications, there is a need for high-power amplifiers (HPA's) that operate in a highly efficient manner. The more efficient the operation of an HPA, the less energy is required to generate a desired output power. Further, higher efficiency HPA's may have size and weight advantages over lower efficiency HPA's capable of providing equivalent output power. In applications such as satellite communications, the size and weight advantages of higher efficiency HPA's may translate directly to cost reductions, for example, due to decreased payload expenses.
HPA's are at their most efficient when operated at or near the saturation region. Operating an HPA at or near its saturation region, however, accentuates non-linearities of the HPA, leading to distortion of the signal, which is then amplified by the HPA. In addition to the distortion introduced into the signal, and related to it, non-linearities of HPA's operated at or near the saturation region cause the power spectral density (PSD) of the output signal to spread outside of its designated frequency band. This phenomenon is known as spectral regrowth. For the case of multiple input signals, the spectral regrowth is caused both due to the nonlinear effects on individual signals and also due to the in band inter modulation (IM) products caused by the interaction among the various signals due to the amplifier nonlinearity. Spectral regrowth occurs even when the input signal is strictly band-limited, for example when the signal is filtered with a square root raised cosine (srrc) filter. Because spectral regrowth can lead to interference, there is a regulatory restriction on the maximum permissible out-of-band PSD relative to the in-band signal PSD.
When the HPA input is comprised of a number of band limited digitally modulated signals, as is the case for example with frequency-division multiplexed (FDM) signals, the spectral regrowth caused by the amplifier nonlinearity results in inter channel interference (ICI) among the various signals. The ICI is especially important when the amplifier input is comprised of a relatively strong signal with relatively high PSD's along with relatively weak signals with relatively low PSD's. In this case even a relatively small spectral regrowth of the strong signal can cause serious ICI to the adjacent weak signals. Limiting the ICI to acceptable limits may require an increased band gap among the adjacent signals thereby reducing the bandwidth efficiency of the system.
Another known technique for dealing with both signal distortion and spectral regrowth due to amplitude non-linearities is to back-off the HPA (i.e., reduce the input signal amplitude such that the HPA is operating farther from saturation). For example, if regulatory requirements regarding spectral regrowth are not met, the back-off of the HPA is typically increased to the level where the spectral regrowth falls within the regulatory requirements. Such an increase in the back-off results in a significant reduction in the output RF power, as well as a reduction in the DC-to-RF power conversion efficiency. Such an increase in the output back off may be required even when the in band distortion is within the acceptable limits, as may be case when error correction coding is used on the digital signal.
Various techniques are known for mitigating non-linear distortion without increasing amplifier back-off. No known techniques, however, effectively addresses spectral regrowth. The known techniques for mitigating amplifier non-linearity generally fall under one of several categories including feed-forward linearizers, feedback linearizers, and pre-distortion linearizers.
A feed-forward linearizer has two loops. The first loop subtracts samples of the input signal from the samples of the amplifier output signal to produce samples of the main amplifier's distortion. The second loop subtracts the amplified sampled distortion from the delayed version of the main amplifier output to obtain the final linearized output. Example feed-forward linearizers are shown, for example, at S. C. Cripps, “Advanced Techniques in RF Power Amplifier Design.,” Artech House, 2002; and J. Vuolevi, “Distortion in RF Power Amplifiers,” Artech House, 2003. The multi-loop feed-forward arrangement, however, is complex to implement, requires a second amplifier that needs to be linear to avoid generating its own distortion terms and results in power loss due to signal combining at the amplifier output.
In feedback linearizers, the amplifier's input and output are detected and low pass filtered. The resulting baseband signals are compared. The error signal is used to control the gain of the amplifier so as to minimize the distortion. This technique suffers from the bandwidth limitation on the amplifier input signal, as the feedback system can respond to frequencies that are much smaller than the inverse of the delay introduced by the amplifier and associated feedback circuitry and thus the technique is limited to relatively narrowband signals.
In pre-distortion linearization techniques, the amplifier input is pre-distorted such that the overall distortion due to the linearizer and amplifier is minimized. The linearizer gain and phase is obtained iteratively for different input power levels. In one known technique, a DSP version of the Cartesian pre-distortion scheme is presented wherein a look-up table that stores the in-phase and quadrature components of the linearizer as a function of the input signal envelope is used for pre-distortion for a set of input signal envelope values. The signal to be amplified is digitized and the sampled values of the signal are modified by interpolated value of the stored in-phase and quadrature components of the linearizer. The correction is limited by the interpolation errors. In another known technique, the stored values are updated according to the amplifier output signal which is also limited due to the interpolation errors. Based on the feedback architecture, a power amplifier linearizer for time division duplex system may be used, wherein a receiving subsystem is shared between the receiver and the power amplifier feedback subsystem resulting in some reduction of complexity for the time division duplex system.
Polar pre-distortion-based architectures are also known, wherein the gain and phase are individually compensated. In most situations, the phase distortion is more important and may be completely eliminated by some architectures. The linearizer gain function is represented by a polynomial of odd degree whose coefficients are obtained by an explicit minimization of the mean squared error between the actual amplifier output gain response and the ideal response which is selected to be linear. The minimization is performed using a gradient algorithm with a power 4 optimization index. The amplitude-to-phase transfer characteristics of the amplifier are directly modeled by a polynomial of the input signal envelope which is then used to compensate for the phase distortion.
In the pre-distortion linearization techniques, the linearized amplifier output is saturated whenever the input signal envelope exceeds the linear range. For band limited input signals this occurs frequently, if the output power back off is kept at a relatively small value, resulting in the out of band spectral growth. The extent of the out of band spectral growth is also a function of the statistical properties of the input signal, for example, it depends upon whether the input signal is a quadrature phase shift-keying (QPSK), an offset quadrature phase shift-keying (OQPSK) signal or a sum of multiple band limited signals. Thus it is desirable to explicitly control the out-of-band spectral growth in addition to the minimization of the in-band distortion.